Page 375 - The Veterinary Laboratory and Field Manual 3rd Edition

344 Susan C. Cork, Willy Schauwers and Roy Halliwell

MoLE For recording purposes, the concentration is
A mole is defined as the amount of substance usually written after the name of the substance,
that contains as many elementary units (atoms, that is, NaCl 1.0 mol/l.
molecules, or ions) as there are carbon atoms in
12 g of the pure carbon C. One millimole is one 2 To make 1 l of physiological saline, that is,
–6
millionth of a mole, that is, 1 × 10 mole. sodium chloride (NaCl), 0.15 mol/l:
MoLEcuLE required mol/l concentration = 0.15
A molecule is an atom or a finite group of atoms
that is capable of independent existence and molecular mass of NaCl = 58.44.
has properties characteristic of the substance of
which it is the unit. Therefore, 1 l NaCl, 0.15 mol/l contains: 0.15 ×
58.44 = 8.77 g of NaCl (solute) dissolved in 1 l
of water (solvent).
MoLEcuLar WEIGHt
Molecular weight is the relative molecular mass
of a molecule of a substance relative to that of an 3 To make 50 ml of physiological saline (NaCl)
12
atom of C taken as 12.000. 0.15 mol/l:
required mol/l concentration = 0.15 molec-
Preparation of molar (mol/l) solutions ular mass of NaCl = 58.44.
and dilutions
To dilute a fluid is to reduce the concentration of Therefore, 50 ml NaCl, 0.15 mol/l contains: 0.15
the dissolved solute in the solvent. The solute is × 58.44 × 50/1000 = 0.438 g of NaCl (solute)
the solid substance that has been dissolved, for dissolved in 50 ml of water (solvent).
example, salt (NaCl), and the solvent is usually
water but may be other fluid substances such as PrEParEd dILutIonS
alcohol. The following examples outline how to 1 To prepare 500 ml sodium hydroxide (NaOH,
prepare solutions and how to dilute them. 0.25 mol/1) from a 0.4 mol/1 solution:
To prepare a molar (mol/1) solution, use the
following formula: X = C × V
S
Required mol/1 solution × molecular mass
of substance = number of grams to be dis- where: C is the concentration of solution
solved in litre of solution required (mol/l); V is the volume of solution
required (ml); S is the strength of the stronger
ExaMPLES solution (mol/l); X is the amount of stronger
1 To make 1 l of sodium chloride (NaCl), 1 mol/l: solution required (ml).

required mol/l concentration = 1 C = 0.25 mol/1 V = 500 ml S = 0.4 mol/1

molecular mass of NaCl = 58.44. 0.25 × 500 = X = 312.5 ml
0.4
Therefore, 1 l of 1 mol/l NaCl contains: 1 ×
58.44 = 58.44 g of the chemical dissolved in 1 l Therefore, measure 312.5 ml NaOH, 0.4 mol/1,
of solvent. and make up to 500 ml with distilled water.

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